Electric vehicles are equipped with a high-voltage main battery for powering a drive motor and a low-voltage sub-battery for powering auxiliary machineries such as headlamps, windscreen wipers, and various electronic devices. The main battery is typically charged by an external power supply for commercial use via a charging circuit, and the sub-battery is typically charged by the main battery via a voltage converting circuit. Although differing in input/output voltage specifications, the charging circuit and the voltage converting circuit are functionally and structurally similar to each other. From the standpoint of reducing size and cost, battery chargers provided with both of the functions of the charging circuit and the voltage converting circuit have been developed (e.g. Patent Literatures 1 and 2).
FIG. 7 is a circuit diagram showing an overall structure of a battery charger pertaining to Patent Literature 1. The battery charger includes: a power supply circuit 1002 connected to an external power supply 1001; a transformer 1003 having a primary winding 1003a connected to the power supply circuit 1002; a secondary-side circuit 1004 connected to a secondary winding 1003b; a smoothing circuit 1006 between the secondary-side circuit 1004 and a main battery 1005; a conduction angle adjustment circuit 1007 connected to a secondary winding 1003c; and a rectifier/smoothing circuit 1008 between the conduction angle adjustment circuit 1007 and a sub-battery 1009.
In a time period for functioning as the charging circuit, a relay 1010 is connected to a capacitor 1011 to charge the main battery 1005 along a route passing through the power supply circuit 1002, the transformer 1003, the secondary-side circuit 1004, and the smoothing circuit 1006. In this time period, the secondary-side circuit 1004 rectifies an AC voltage induced in the secondary winding 1003b by using body diodes of field-effect transistors 1004a and 1004b. The conduction angle adjustment circuit 1007 adjusts a conduction angle of an AC voltage induced in the secondary winding 1003c so that the AC voltage is supplied to the rectifier/smoothing circuit 1008. As a result, the sub-battery 1009 is also charged.
On the other hand, in a time period for functioning as the voltage converting circuit (during driving the vehicle), the relay 1010 is connected to a choke coil 1012 to disable a series circuit of the choke coil 1012 and the capacitor 1011 within the smoothing circuit 1006. The sub-battery 1009 is thus charged along a route passing through the main battery 1005, the smoothing circuit 1006, the secondary-side circuit 1004, the transformer 1003, the conduction angle adjustment circuit 1007, and the rectifier/smoothing circuit 1008. In the time period for functioning as the voltage converting circuit, the secondary-side circuit 1004 converts a DC voltage from the main battery 1005 into an AC voltage.